Today's cellular communication systems provide not only voice services, but also mobile broadband services all over the world. As the number of applications for cell phones and other wireless devices continues to increase, and consume greater amounts of data, an enormous demand for mobile broadband data services is generated. This requires telecom operators to improve data throughput and maximize the efficient utilization of limited resources wherever and whenever possible.
In response to the fact that the spectrum efficiency for point-to-point links is already approaching its theoretical limit, the telecom industry has introduced the concept of a multi-layered network in order to fulfil the growing demands for mobile broadband data services. Generally, a multi-layered network consists of several layers of base stations that provide or enable different physical resources such as carriers, transmit power, etc. A heterogeneous network (HetNet) is one example of a typical two-layered network in which a macro layer consisting of high transmit power base stations (macro layer nodes) is complemented with a low transmit power node (LPN) layer using at least one common carrier. Another example of a heterogeneous network comprises a macro layer complemented with a layer of LPN nodes that provide communications using a different frequency carrier than the macro layer nodes.
One consequence of deploying a multi-layered network, however, is that the density of cell sites must be significantly increased. For example, the inter-site distance between small cells can be 20 meters or less, as compared to a distance of hundreds of meters between macro cells. The hyper-dense deployment requires that user equipment (UE) can discover the surrounding cells in time. However, the legacy cell discovery mechanism that detects synchronization channels (PSS and SSS) is designed and optimized for macro deployment, not for a hyper-dense cell deployment.
The current Channel State Information Reference Symbol (CSI-RS) may be extended for small cell discovery purposes. According to current 3GPP specifications (Rel-11), CSI-RS for one port is one pair of resource elements (RE) in each resource block (RB) in one sub-frame. The term “RB” is used herein to refer to the region of time-frequency plane containing 12 continuous sub-carriers in the frequency domain and one sub-frame in the time domain.
FIG. 1 illustrates one embodiment of a current CSI-RS configuration 100. In the illustrated CSI-RS, there are in twenty pairs of REs 102 defined in each RB 104 for carrying CSI-RS symbols corresponding to twenty configurations. Each pair contains two adjacent REs and a cover code with a length of two is applied onto the pair in order to allow two ports of CSI-RSs to share the same physical REs.
In each configuration, multiple ports {1, 2, 4, 8} can be defined with numbering from port 15 to 22 and a UE may assume that all of the antenna ports from one CSI-RS configuration are quasi-co-located. In order to reduce signaling overhead, a common reference RE pair is defined for a different port setup for the same configuration and a fixed offset in the frequency domain applied onto the common reference RE pair for different ports. An example configuration is shown in Table 1.
TABLE 1Fixed Offset for each Antenna PortPort Setup (Normal CP)Port Setup (Extended CP)Port Nbr.1248124815000000001600000017−6−6−3−318−6−6−3−319−1−620−1−621−7−922−7−9
A UE can perform measurements (such as channel estimation, received power estimation, etc.) on each antenna port separately. However, the UE may perform averaging or other filtering across the same RE pair in different RBs. Current CSI-RS configurations can be used for small cell discovery purposes, but can only be applied to UEs in RRC_CONNECTED mode. The serving cell creates a target small cell list intended to be discovered by a specific UE, and decides the CSI-RS configurations for the list. The serving cell informs the corresponding small cells via an X2 interface and informs the UE of the CSI-RS configurations via signaling messages. After the UE receives a certain CSI-RS configuration, the UE finds the accurate timing of the target CSI-RS symbols within a predefined time window by correlating the locally generated CSI-RS samples to the received signal.
There are two main issues for applying the legacy CSI-RS for cell discovery purpose: (1) In a non-quasi-co-located scenario, if only one antenna port in one CSI-RS configuration is used for cell discovery, according to the current 3GPP specifications (Rel-11), only antenna port 15 with the cover code (+1,+1) is used. The other code (+1,−1) applied on the same set of resources is wasted, because it can be used by another small cell as the discovery signal; and (2) Due to the nature of CSI-RS and that CSI-RS RE pairs are repeated once in each RB, there are many peaks in its auto-correlation function and the distance between two adjacent peaks is around 5.56 microseconds (μs) as illustrated in FIGS. 2A-2B.
If the searching window size is more than 5.56 μs when the UE performs refined time synchronization, then there will be two peaks with the similar magnitude in the results, even if there is no noise in the signal. This peak ambiguity problem introduces severe ISI (inter-symbol interference), degrading the detection and measurement performance of CSI-RS. Multiple antenna ports for one CSI-RS configuration according to the current specifications cannot solve the peak ambiguity problem, as the above mentioned operations are performed independently on each antenna port.